R some very lispy meta-programming features. Very briefly, if I'm a function (go with me here) `foo <- function(x,y){whatever}`,and someone calls me, `foo(bar,baz)` I get to know that `baz` is "bound" to `y`. The caller doesn't even have to have a `baz` in scope; substituting `y` for `baz` is done lazily, if the function never asks for the actual value of `y`, there's no problem. You can read more about this here http://adv-r.had.co.nz/Computing-on-the-language.html
R gives you the ability to capture un-evaluated expressions, manipulate them, and then evaluate the manipulated versions. This ability lets you construct some fairly powerful DSLs and language extensions.